Support promotion rule override in vttablet

[timvaillancourt]

Description

This PR implements #16377 by adding a --promotion-rule topodatapb.PromotionRule flag to vttablet (and indirectly vtcombo) to express an override to the promotion rule of a single tablet

  --promotion-rule topodatapb.PromotionRule                          Sets the override Promotion Rule for this tablet. Possible values 'none', 'neutral', 'prefer', 'prefer_not' and 'must_not'. Use 'must_not' with caution as it can lead to issues with reparenting. (default none)

This is an "advanced" feature that comes with some risk if promotion rules like must_not are deployed too widely; this risk falls "on the user" and a no-override deployment is recommended (the Durability Policy calls the shots for tablets consistently)

In a subsequent release I would like to phase-out the promotionrule.CandidatePromotionRule type in favour of the new topodatapb.PromotionRule type (added in this PR). I believe waiting 1 release will ensure proto-level downgrade/upgrade compatibility

Related Issue(s)

#16377

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Tests were added or are not required
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• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 12 lines in your changes missing coverage. Please review.

Project coverage is 68.67%. Comparing base (7a737f4) to head (014728d).
Report is 268 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/topo/topoproto/flag.go	0.00%	10 Missing ⚠️
go/vt/vtctl/reparentutil/durability.go	80.00%	1 Missing ⚠️
go/vt/vttablet/tabletmanager/tm_init.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16344      +/-   ##
==========================================
+ Coverage   68.61%   68.67%   +0.06%     
==========================================
  Files        1544     1548       +4     
  Lines      197993   199119    +1126     
==========================================
+ Hits       135848   136747     +899     
- Misses      62145    62372     +227     

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[github-actions]

This PR is being marked as stale because it has been open for 30 days with no activity. To rectify, you may do any of the following:

• Push additional commits to the associated branch.
• Remove the stale label.
• Add a comment indicating why it is not stale.

If no action is taken within 7 days, this PR will be closed.

[timvaillancourt]

I'm going to close this for now in favour of using dynamic tablet tags + our custom durability policy

#16857 will implement dynamic tablet tags 🚀